A configuration has been disclosed that performs receiving processing by means of discrete time direct sampling of a high-frequency signal with the aim of achieving small size and low power consumption of a radio receiver and integrating the analog signal processor and digital signal processor (e.g. see Patent Document 1).
An example of the configuration of a discrete time direct sampling circuit using conventional discrete time processing and its sampling and filter processing operations will be described using FIG. 1. In FIG. 15, discrete time direct sampling circuit 1500 is provided with: differential voltage-to-current convertor 1501 that converts a received radio frequency (“RF”) signal to differential current signals and outputs positive-phase analog RF current signal 1511 and negative-phase analog RF current signal 1512; sampling mixer 1502 that is comprised of, for example, a plurality of mixer switches, and that samples differential analog RF current signals that are received as input, based on a positive-phase local frequency signal (hereinafter “positive-phase LO signal”) and negative-phase local frequency signal (hereinafter “negative-phase LO signal”) having phases shifted by half the period to each other; electric charge integration processor 1503 that charges and integrates electric charges supplied by electric currents outputted from sampling mixer 1502; and control signal generator 1504 that generates local signals used for sampling and control signals used for integrating, charging and resetting an electric charge, for sampling mixer 1502 and electric charge integration processor 1503.
FIG. 2 illustrates a timing chart of control signals generated in control signal generator 1504. Here, a case commonly referred to as “zero IF reception” or “low IF reception,” will be explained as an example. In the case of zero IF reception, a positive-phase LO signal and negative-phase LO signal are supplied to switch gates in sampling mixer 1502, and their frequency is substantially the same as the frequency of the analog RF signal. Also, signal D is supplied to the gates of integration switches 15031 and 15032 in electric charge integration processor 1503. Signal R is supplied to the gates of reset switches 15033 and 15034 in electric charge integration processor 1503.
An example given here is designed such that signal D turns “on” integration switches 15031 and 15032 over a time period matching six LO signal samples, signal R turns “on” reset switches 15033 and 15034 over a period of time matching one LO signal sample, and voltages proportional to the amounts of electric charge integrated and charged in capacitors are read at timings while the switches are turned “on” with signal D and signal R.
The operations of discrete time direct sampling circuit 1500 will be explained below. Differential voltage-to-current convertor 1501 converts an analog RF signal received as input, into differential analog RF current signals, and outputs the positive-phase and negative-phase analog RF current signals to sampling mixer 1502. The differential analog RF current signals are subjected to sampling through switches 15021 to 15024 in the sampling mixer using LO signals having substantially the same frequency as the analog RF signal. In electric charge integration processor 1503, the differential analog RF current signals, each sampled in sampling mixer 1502, are charged in integration capacitors 15035 and 15036, via integration switches 15031 and 15032, over a period of time of six LO signal samples. By this means, the electric charges supplied by the differential analog RF current signals are integrated over a duration of a period of six LO signal samples. Voltages proportional to the amounts of electric charges integrated and charged in integration capacitors 15035 and 15036 are read out from output ports 1513 and 1514 as discrete time analog signals during the holding period.
Here, the discrete time analog signals read out from output ports 1513 and 1514 are subjected to filter processing of two lowpass characteristics: a lowpass filter characteristic having a SINC function characteristic acquired by performing integration over a period matching approximately half of the period of the local signals; and a discrete time, FIR (Finite Impulse Response) lowpass filter characteristic acquired by adding frequency-domain discrete signals of the LO signals acquired as above over six samples, and their overall characteristics are shown in FIG. 3. Here, in FIG. 3, the frequency represented by the horizontal axis is normalized by local frequencies, and the gain represented by the vertical axis is normalized by the maximum value.
As described above, discrete time direct sampling circuit 1500 performs filter processing with the bandpass characteristic shown in FIG. 3 and acquires discrete time analog signals whose sampling frequency is decimated (punctured) to ⅙ of the local signal.
Patent Document 1: National Publication of International Patent Application No. 2003-510933